1. Field of the Invention
The present invention relates to a ladder filter including a plurality of elastic wave resonators. In more detail, the present invention relates to a ladder filter including a series arm resonator and a parallel arm resonator, which include elastic wave resonators, located on one piezoelectric substrate, and a duplexer including the ladder filter, defining at least one band pass filter.
2. Description of the Related Art
In the past, ladder filters have been widely used as band pass filters for mobile phones. In addition, in this type of ladder filter, so as to achieve miniaturization, a series arm resonator and a parallel arm resonator are configured using surface acoustic wave resonators.
For example, in Japanese Unexamined Patent Application Publication No. 2000-201052, an example of such a ladder filter is disclosed. FIG. 8 is a plan view schematically illustrating this ladder filter.
In the ladder filter 1001, series arm resonators S101 to S103 and parallel arm resonators P101 to P102 are configured on a piezoelectric substrate 1002. Each of the series arm resonators S101 to S103 and the parallel arm resonators P101 to P102 has an IDT electrode and reflectors disposed on both sides in the elastic wave propagation direction of the IDT electrode. Here, the diagrammatic representation of the reflector is omitted.
So as to advance miniaturization, each of the above-mentioned resonators is formed on the one piezoelectric substrate 1002.
However, in the ladder filter 1001, on the piezoelectric substrate 1002, the series arm resonators S101 to S103 are spaced from each other with a predetermined distance therebetween. Accordingly, spaces A are provided between the series arm resonators S101 and S102 adjacent to each other and between the series arm resonators S102 and S103 adjacent to each other. In addition, the parallel arm resonators P101 and P102 have been disposed lateral to the spaces A. More specifically, so as to achieve miniaturization, a portion of the parallel arm resonator P101 has been disposed between the series arm resonator S101 and the series arm resonator S102. In the same way, a portion of the parallel arm resonator P102 has been disposed between the series arm resonator S102 and the series arm resonator S103.
However, in such a configuration, not only the above-mentioned spaces A have existed but also a space B has also existed between the parallel arm resonator P101 and the parallel arm resonator P102. The space B is located outside the series arm resonator S102 in the elastic wave propagation direction thereof. In the same way, since the parallel arm resonators P101 and P102 are disposed in the manner described above, a space B has also occurred outside each of the series arm resonators S101 and S103 in the elastic wave propagation direction thereof.
Accordingly, although in order to achieve miniaturization, a portion of the parallel arm resonator P101 or P102 has been caused to be disposed in between the series arm resonators S101 and S102 adjacent to each other or the series arm resonators S102 and S103 adjacent to each other, the spaces A and B have been still large. Accordingly, it has been difficult to further promote miniaturization.